what do the wheels symbolize?

The wheels that the children use working in factories become a metonymic symbol of the degradation of industrial labor, in particular its repetitiveness and noisiness. Through the symbol of the wheels, Browning is able to vividly portray the children's struggles through a physical, literal lens—for instance, they express their feelings of being silenced and ignored through the idea that the noisy wheels might drown out their voices. The wheels' circularity and endless turning also suggests that child labor is a cycle, repeating and self-perpetuating just like the technology that the children use.